1. Field of the Invention
The present invention relates generally to sensor measurements in automobile control systems and, more particularly, to a system for enhancing the precision of an analog sensor reading in an automobile control system.
2. Discussion of Related Art
Current automobile engines are internal combustion engines that use a mixture of fuel and air to generate their driving power. Complete fuel combustion produces only carbon dioxide and water as its products; however, the conditions within an engine do not correspond to the idealized requirements necessary to produce complete combustion. Incomplete combustion produces other products that may include: carbon monoxide, hydrogen gas, hydrocarbons, nitrogen gas, oxygen gas and various nitrous oxides. Some of these gases are commonly found in the atmosphere and pose few or no health risks. Others can be highly toxic, and their emissions must be reduced.
The United States and many other countries have strict standards regulating the emissions from automobiles. Catalytic converters transform toxic chemicals into safer compounds. They convert CO; H.sub.2 and HC into CO.sub.2 and H.sub.2 O and also convert nitrous oxides into nitrogen gas and oxygen gas before these gases are emitted from the automobile; however, catalytic converters are not completely efficient, and some of the toxic byproducts of incomplete combustion are not converted into less harmful substances before their emission into the atmosphere. The higher the efficiency of the catalytic converter, the more toxic gases are converted into safer forms before they are emitted into the atmosphere. The efficiency of a catalytic converter is directly related to the composition of its intake gases, and the composition of the intake gases is determined by the combustion conditions, including the fuel-air mixture ratio used in the engine.
The mixture of fuel and air used in the combustion chamber of an engine is regulated through a feedback mechanism. A sensor is placed in the exhaust manifold, and it measures the oxygen content in the expunged gases. The oxygen content of the combusted mixture can be used to determine where in relation to the stoichiometric operating point the engine is currently operating. Typically, the operating point of the engine is called the stoichiometric fuel-air ratio, and this corresponds to the point where the exact quantity of fuel needed for completed combustion is added to the air flow. The stoichiometric point has the most efficient catalyst operation and produces the least amount of toxic byproducts. The varying operating characteristics of the vehicle will change the efficiency of the combustion process and will require altering the current fuel flow to keep the engine operating at or near the stoichiometric point. The oxygen sensor output is used to optimize the fuel-air ratio fed into the engine. Optimizing the fuel-air mixture entering into the engine changes the combustion conditions and achieves more complete combustion, thereby operating the engine closer to the stoichiometric point.
The oxygen sensors used in most vehicles provide a voltage output based on the amount of oxygen in the combustion product. This information is input into an analog-to-digital converter (A/D) and the output of the A/D is fed into a digital microprocessor. The microprocessor controls the fuel-air ratio and constantly adjusts the mixture entering the combustion chamber in order to keep the engine operating near the stoichiometric point. Constant adjustment is required, because changing engine and environmental conditions alter the efficiency of the combustion process, even for a constant fuel-air mixture ratio. The voltage output of the oxygen sensor varies with the amount of oxygen found in the combustion products. The variation in voltage directly around the stoichiometric operating point is large and away from the stoichiometric point the variation is small, even for a large oxygen content change, as shown in FIG. 2. The non-linear voltage dependence makes measuring the prevailing operating point difficult since the variation around the stoichiometric point during the engine's normal operation is generally large. The normal functioning of the engine will include operation in regions away from the stoichiometric point, where when the variation in exhaust oxygen content is large only a small voltage change in the sensor occurs. This small voltage change requires a precise system to detect the changes and to determine the exact operating point of the engine. The precision of present measurement systems is only accurate enough to allow a precise determination of the operating point directly around the stoichiometric value. When non-trivial variations from the stoichiometric point occur, the difficulty in obtaining an exact measurement due to the lack of precision in reading the analog voltage change essentially transforms the oxygen sensor into a switch sensor; the engine is operating above or below the stoichiometric point but no data is provided as to the exact oxygen content. This large variation and resulting imprecise measurement does not allow an adequate fuel-air ratio adjustment to efficiently drive the engine back to the stoichiometric operating point.
A more precise method of measuring the voltage output from an analog sensor in an automobile control system is needed. The method must increase the precision of the analog readings and must allow an accurate determination of the operating point of the engine over the sensor's entire range of output values.